This invention is related to MEMS devices. More particularly, this invention is related to maintaining the state of a MEMS device in the event of a power failure.
Modern communications systems require a level of robustness that protects the state of the optical switches from being lost in the event of a power failure. Recently, microelectromechanical systems (MEMS) devices have been developed for optical switching. MEMS devices are miniature mechanical devices manufactured using the techniques developed by the semiconductor industry for integrated circuit fabrication. MEMS optical switches typically include an array of mechanically actuatable mirrors that deflect light from one optical fiber to another. The mirrors are configured to translate or rotate into the path of the light from the fiber. Mirrors that rotate into the light path generally rotate about a substantially horizontal axis, i.e., they xe2x80x9cflip upxe2x80x9d from a horizontal position into a vertical position. MEMS mirrors of this type are usually actuated by magnetic interaction, electrostatic interaction, thermal actuation or some combination of these. The MEMS mirrors may be retained in the xe2x80x9cupxe2x80x9d position by an electrostatic clamping voltage. In the event of a power failure, the clamping voltage may be lost and any MEMS mirrors that were clamped may return to the xe2x80x9cdownxe2x80x9d position under the influence of mechanical restoring forces. In this manner, the state of the switch may be lost in the event of a power failure.
The problem is illustrated through an example shown in FIG. 1, which depicts a schematic diagram of a MEMS apparatus according to the prior art. The depicted apparatus generally includes a MEMS optical switch 100. The optical switch 100 has a substrate 102, and a moveable element 104 moveably coupled to the substrate 102. The moveable element 104 may be one of several such moveable elements that are moveably coupled to the substrate 102. The moveable element 104 moves between a horizontal xe2x80x9cOFFxe2x80x9d position (shown in phantom) and a vertical xe2x80x9cONxe2x80x9d position. In the xe2x80x9cONxe2x80x9d position, the moveable element 104 is retained against a top chip 106. In this example, the top chip 106 is electrically isolated from the substrate 102, and all other MEMS elements, and a clamping voltage, e.g., +40 V, is applied between the moveable element 104 and the top chip 106. In the apparatus shown in FIG. 1 the clamping voltage difference is supplied by a high voltage source, such as a DC-DC converter 130 and a high voltage driver 120. The high voltage driver 120 is essentially an electronic switch for addressing and selectively coupling a plurality of moveable elements 104 to the voltage potential output by the DC-DC converter 130 or to ground. In this example, the output of the DC-DC converter is also coupled directly to the top chip 106. Thus, the top chip 106 sustains a clamping voltage as long as power is supplied to the DC-DC converter 130. The high voltage driver 120 may be controlled by a microcontroller 110, e.g., a PIC microcontroller to set a voltage potential for each movable element 104 configured in an optical cross-connect switch matrix. Depending on the required state of the switching element 100, a voltage difference may exist between the moveable element 104 and one or more clamping structures. The clamping structure may clamp the movable element in a state and may also provide a mechanical stop to accurately align and fix the movable element in the required state. A top chip may be assimilated herein for purposes of examples shown, as an electrostatic clamping surface having a global mechanical stop to accurately align the movable element in the ON state. In this example, the top chip 106, charged to some electrostatic potential (Vclamp), provides the mechanical stop and clamps the moveable element 104 when the moveable element 104 is electronically connected to zero voltage (ground) through the high voltage driver. Alternatively, when the output of a high voltage driver coupled to the movable element 104 is set to Vclamp through the high voltage driver 120, no clamping voltage difference is present between the top chip 106 and the moveable element 104 and thus the moveable element 104 is allowed to fall back to the OFF state. It is also important to note in this example that in the clamped state, a small insulating gap, such as an air gap, is maintained between the top chip and the moveable element in order to maintain electrical isolation between the two surfaces.
In the event of a power failure in the example shown, the microcontroller 110 no longer receives the logic voltage Vcc and, therefore, can no longer control the high voltage driver 120. Although the top chip 106 is electrically isolated from the other MEMS elements, the DC-DC converter 130 and high voltage driver 120, both sharing the same circuit node as the top chip 106, may be resistively coupled to ground. The coupling of the top chip 106 to these circuits causes charge to leak from the top chip 106 to ground. If the leakage of charge is sufficiently large, the voltage difference between the top chip 106 and the moveable element 104 will quickly be reduced to a level insufficient to retain the moveable element 104 against the top chip 106. The moveable element 104 then returns to the xe2x80x9cOFFxe2x80x9d position interrupting any optical signal that may be deflected by the moveable element 104. Even when power is restored, the state of the MEMS device 100 will not be recovered since the clamping voltage does not actuate the moveable element 104.
Thus, there is a need in the art, for a method of maintaining the state of a MEMS device in the event of a power failure and an apparatus for implementing such a method.
The disadvantages associated with the prior art are overcome by a method and apparatus for maintaining the state of a MEMS device in the event of a power failure. The MEMS device generally has one or more MEMS elements moveably coupled to a substrate and a clamping surface that may be electrically isolated from all other MEMS elements. According to the method, an adequately sized charge storage device is connected between the clamping surface and an electrical ground. A clamping voltage applied between a clamping surface and at least one MEMS element retains the at least one MEMS element against the clamping surface. In the event of a power failure, all potentially leaky circuit paths to ground are isolated from the clamping surface, with the exception of the charge storage device that serves to maintain the electrostatic clamping voltage.
The apparatus generally comprises a charge-storing circuit, e.g., a capacitive circuit or battery permanently connected between the clamping surface and an electrical ground and an isolator element electrically connected between the clamping surface and all other circuits sharing the same node as the clamping surface (e.g., a top chip). The isolator element is configured to electrically isolate all potentially leaky circuit paths from the clamping surface in the event of a power failure. The isolator element may include an opto-isolator, diode or other circuit capable of providing low-leakage electrical isolation.